Buttonholes produced on double-lock-stitch buttonhole-sewing machines are formed as so-called "raised buttonholes" for washable garments such as shirts, blouses and the like.
To produce such a buttonhole it is necessary to work with a relatively high upper-thread tension (about 300 g with higher and lower values possible according to the type of thread and material) and a relatively low lower-thread tension (about 30-40 g) so that the interengaged loops of the upper and lower threads are drawn to and lie upon the upper surface (outer surface) of the fabric.
The resulting buttonhole stitch gives the appearance of a manual sewing operation.
The aforedescribed method is, however, disadvantageous in two respects. Firstly, with the production of the buttonhole, a relatively large length of lower thread and a substantially shorter upper thread are used. Since the thread quantity for the lower thread is limited, e.g. by the capacity of the bobbin lying under the stitchplate, a single filling of the bobbin, depending upon the type of thread, can be used for only about one hundred shirt buttonholes. This means that for an average output of five thousand buttonholes per day, fifty bobbin changes may be required.
Secondly, because of the low consumption of the upper thread, which is used in a length only of about twice the length of the perimeter of the buttonhole, this relatively short length of thread must pass through the fabric and the eye of the needle some two hundred times back and forth. As a result, this length of upper thread is subjected to a high degree of wear so that the output capabilities of the machine, as a result of this excessive thread wear, cannot be fully utilized.
These disadvantages can be overcome by a conventional double-lock-stitch buttonhole-sewing machine in which the stitching mechanism, namely, the needle and gripper, are reversed in orientation. Thus, the gripper and the lower-thread bobbin are disposed above the stitchplate which the needle carrying the upper thread is disposed beneath the stitchplate. If then one operates with a high bobbin-thread tension and a low needle-thread tension, the loops of the two threads interengage at the upper side of the fabric but with the advantage for the sewing operation that relatively little bobbin thread and a larger amount of needle thread is consumed.
However, a buttonhole-sewing machine for carrying out this process is materially different from the usual commercial double-lock-stitch sewing machine and must be designed and manufactured separately.
This disadvantage can be obviated when, in accordance with the principles described in German Pat. No. 739,559, a modified technique is used.
In accordance with the method of this patent, the workpiece is turned face down upon the stitchplate of a double lock stitch buttonhole-sewing machine and the upper-thread tension is relatively low by comparison with the lower-thread tension, i.e. the lower-thread tension is greater than the upper-thread tension.
As a result, the interengaging loops of the upper and lower threads lie upon the underside of the workpiece, i.e. upon the outer face of the fabric which has been turned down to this end. Consequently, a large length of upper thread and relatively little lower thread is used to achieve the so-called raised buttonhole.
While this system has been found to be highly advantageous, there arises a problem at the end of the buttonhole-sewing cycle with respect to the fastening stitch which must form a type of knot. In the system of this patent, the knot appears upon the outer face of the garment, i.e. upon the downwardly turned surface of the fabric workpiece.
It is well-known that this system does not allow the fastening knot to be drawn into the fabric or to the reverse side therof as can be achieved with the previously described stitching systems. Thus, although the method and apparatus of the aforedescribed are highly advantageous for the stitching of the length or perimeter of the buttonhole to produce this so-called raised buttonhole, the disadvantage remains that the knot or final stitch lies on the exposed or outer face of the fabric.